Radiation Oncology: Radiobiology and Radiation Physics
Radiobiology
Ionising radiation injures tissue through the 4 Rs: repair of sublethal damage, reassortment across the cell cycle, repopulation and reoxygenation. Radiosensitivity peaks in late G2/M and is lowest in late S phase.
The linear-quadratic model gives survival as exp(-alpha d - beta d squared). The alpha/beta ratio characterises tissue behaviour: about 10 Gy for early-responding tumours and early effects, and about 1.5 to 3 Gy for late-responding tissues such as prostate and central nervous system. This explains why hyperfractionation spares late effects and why hypofractionation works in prostate cancer.
Biologically effective dose: BED = n times d times (1 + d divided by alpha/beta). Worked example: 60 Gy in 30 fractions of 2 Gy gives BED10 = 72 Gy10 and BED3 = 100 Gy3.
The oxygen enhancement ratio is about 3 for X-rays; high-LET radiation (carbon ions, alpha particles) has a higher relative biological effectiveness and less oxygen dependence. Protons show a Bragg peak, depositing dose sharply at a defined depth with a distal falloff, sparing tissue beyond the target.
Physics
Dose units: Gray (1 Gy = 1 J/kg absorbed dose); sieverts for equivalent and effective dose. Clinical photon beams are megavoltage. Isodose distributions, dose-volume histograms and inverse planning underpin IMRT, VMAT and SBRT; accuracy depends on immobilisation and image guidance.
Traps
Acute effects occur within 90 days in rapidly dividing tissues (mucositis, skin desquamation, diarrhoea, myelosuppression); late effects occur months to years later in slowly dividing tissues (fibrosis, myelopathy, xerostomia, second malignancy). Never confuse the two.